Bilateral amputees constitute a relatively small proportion of lower limb amputees. Despite this, a bilateral amputation is a much more significant disability than a unilateral amputation, particularly in the case of transfemoral amputation. Specifically, in the case of unilateral amputation, an amputee is able to compensate substantially with his or her sound side to address biomechanical deficiencies in the prosthetic limb. For example, relative to healthy individuals, unilateral transfemoral amputees rely disproportionately on their sound side leg for providing the net power output required for stair ascent, slope ascent, and for sit-to-stand transitions, all of which require net positive power at the knee and/or ankle joints. Unilateral transfemoral amputees also provide significant compensatory effort with their sound side leg in activities which do not necessarily require net positive power, such as level walking, slope and stair descent, standing (particularly on uneven terrain), and stand-to-sit transitions. For example, “heel hiking” is a common compensatory action observed during sound-side stance in level walking. Heel hiking elevates the amputee's center of mass, in order to increase the swing-side clearance between the prosthetic foot and ground, in order to decrease the likelihood of scuffing or stumbling with the prosthetic leg.
Unlike unilateral amputees, bilateral transfemoral amputees are not able to compensate with a sound-side limb for deficiencies in gait biomechanics on the prosthetic side, and in particular are not able to compensate similarly for deficiencies in power generation.